A heavily armed man who was arrested this week for allegedly plotting to attack Norco College had at least one accomplice who has not yet been identified by law enforcement, according to court records filed on Thursday.

A criminal suspect can only be charged with conspiracy if evidence suggests they worked with another person while planning a crime.

Court documents state that the conspiracy charge is based, at least in part, on the allegation that McBain conveyed his threat to kill on Facebook Messenger, which is an app used for online private communication.

"That is still under investigation and the co-conspirator or co-conspirators have not yet been identified," said John Hall, a spokesman for the Riverside County District Attorney's Office.

McBain was arrested Monday after the Riverside County Sheriff’s Department was notified about a series of social media posts where he said he intended to shoot people at the college campus, according to a news release. The threats were discovered only five days after a gunman in Florida killed 17 students at Stoneman Douglas High School, the latest in a relentless rhythm of mass shootings in the U.S.

In the McBain case, authorities deemed his online threats credible, so the sheriff’s department got a search warrant for his home on Trail Street in Norco. While serving the warrant, deputies found a loaded AR-15, two loaded handguns and a “large amount of ammunition” in McBain’s bedroom, police said.

Court records say McBain had 510 rounds of ammunition.

This loaded Ar-15 rifle and two loaded handguns were found in the Norco home of suspect Jacob Ryan McBain on Monday.(Photo: Riverside County Sheriff's Department)

Authorities did not initially identify the campus that McBain had targeted, but the school was later identified in court records as "Riverside Community College in the City of Norco," which is more commonly know as Norco College The sheriff's department said in a press release that it alerted the school to the McBain investigation, even though he was arrested on Monday, a federal holiday when no students were at campus.

Classes are continuing as normal at Norco College, but campus security has been increased, said campus spokeswoman Diana Mesa. Earlier this week, the college released a statement saying student safety was of “critical importance.”

“Together with (the Corona Norco Unified School District,) our school has zero tolerance for any behavior that threatens the safety of our school,” said Norco College President Bryan Reece in the statement. “The consequences for anyone who chooses to engage in behavior targeting the safety of our schools are severe.”

McBains’s arrest occurred at time when the Parkland massacre has a reawakened a national concern about school shootings, and campus throughout Southern California have seen a spree of scares. A Fontana man was arrested Tuesday after he allegedly threatened gun violence at a Riverside after-school program, and teenagers were arrested in both Long Beach and Whittier after they were overheard making threats in separate incidents. Rumors also circulated online about threatened violence at Poly High School in Riverside, but police said the threats were unfounded.

The online threats that launched the investigation do not appear to be visible on McBain’s current Facebook page. Instead, the page consists mostly of memes – many showing an apparent fondness for guns and the AR-15 in particular. In February, McBain posted a video for the Barracuda intruder defense system, a door locking device that is designed to prevent school shootings.

McBain is being held at the Riverside jail on $1 million bail. He made his first appearance in court during a brief hearing on Thursday afternoon, at which point he was appointed a public defender. McBain's attorney, Douglas Redden, did not immediately respond to a request for comment.

His full charges are nine felonies: Conspiracy to commit a crime; two counts of criminal threats of death or great bodily injury; two counts of convicted felon and narcotic addict in possession of a firearm; three counts of illegal firearm possession and one county of possession of an assault weapon

That last charge stems from McBain's AR-15, a semi-automatic military-style combat rifle commonly used in school shootings. The rifle has become a centerpiece of the national gun control debate, with many arguing that this weapon of war has no place in civilian hands. In California, the AR-15 is considered an assault rifle.

Desert Sun reporter Colin Atagi contributed to this story.

Investigative reporter Brett Kelman can be reached at 760 778 4642 or by email at brett.kelman@desertsun.com. You can follow him on Twitter @tdsBrettKelman.